Such a catalyst system for trimerisation of olefins is known from EP-A-0608447 and consists of a combination of a transition metal source, a pyrrole containing compound and a metal alkyl in an electron donor solvent. The transition metal source consists of a chromium, nickel, cobalt, or iron compound, preferably a chromium compound is used.
Because chromium compounds are highly toxic, and therefore need special handling precautions, a catalyst system for the trimerisation of olefins, which is not based on a chromium compound is needed.